A “micro-object” should be understood to be any object having at least one dimension less than or equal to 10 μm. A “nano-object” should be understood to be any object having at least one dimension less than or equal to 1 μm or even, according to a more restrictive definition, to 100 nm (in which case, between 100 nm and 1 μm, the object will be called “sub-micrometric”).
Generally, it is difficult to position and accurately orient micro-objects, and even more so nano-objects, because the observation thereof is not easy. That stems both from their low intrinsic visibility and to the bulk due to the planar support and to the macroscopic elements (micromanipulators) allowing them to be manipulated.
One example of these difficulties is encountered in the field of scanning probe microscopy (SPM), which covers various techniques such as tunnel-effect microscopy (STM, for “Scanning Tunneling Microscopy”), atomic force microscopy (AFM) and electrochemical microscopy (SECM, from “Scanning Electro-Chemical Microscopy”).
A scanning local probe microscope, or simply local probe microscope—explores a surface point by point along a line (position of the probe on an axis x), then line by line (position of the probe on an axis y, at right angles to the axis x), in order to produce an image S(x,y), where S is a signal relating to an interaction between the probe and the surface. The nature of this interaction conditions the type of microscopy considered: in atomic force microscopy, the surface is observed by measuring the force between the probe and the surface; in scanning tunneling microscopy, it is observed using the measurement of the tunnel current; and in scanning electrochemical microscopy, the probe is immersed in an ion solution, and it is the current deriving from the electrochemical processes on the surface, in the solution or at the probe, which makes it possible to image the sample.
Whatever the technique used, the interaction is always greatly dependent on the distance h between the probe and the surface being explored. Moreover, with constant distance h, this interaction depends on at least one local property p of the surface (for example the conductivity in STM), so that the signal S is a function of x, y, h and p. A number of local properties can simultaneously affect the signal S (for example the local conductivity of the surface and the Van der Waals interaction between the point of the STM and the surface), that can be taken into account with a vector notation p. Similarly, geometrical factors other than h can also affect S, and in particular the inclination of the axis of the probe relative to the normal to the surface, which is an element that is particularly important in the case of SECM. To take account of this, here again it is possible to use a vector notation, h. In total, the signal delivered by the point can be denoted S(x,y, h, p).
Generally, an SPM microscope has two main operating modes:
In the first, called “constant distance”, the height h is corrected during the scanning in order to keep the signal S constant, and the extracted information h(x,y) describes the reliefs of the surface with a resolution which depends on the fineness of the point.
In the second, called “constant height”, the probe is made to move in a plane z(x,y) and every effort is made generally to keep this plane as parallel as possible to the surface of the sample so as to keep the height h(x,y) approximately constant, and the variations of the signal S due to the other parameters are recorded, making it possible to map one or more properties p(x,y) of the surface.
Since the interaction between the point and the surface depends on both h and p, the separation of the two sets of parameters is always difficult. One approach for overcoming this difficulty is to record not one but several measurements of S at each point (amplitude and phase in AFM, for example). To obtain these multiple measurements, one of the control parameters, such as, for example, the potential of the probe in electrochemical microscopy, must be varied at each point. For example, it is possible to vary the distance h between the point and the surface at each point x,y or modulate this distance at a frequency much greater than the scanning frequency. This approach is particularly difficult to implement in the case of SECM, because the measured quantity (an electrical current) is more sensitive to the properties of the surface and of the electrolyte studied than to the distance h, whereas, in most of the other SPM techniques, the measurement S depends very greatly on h and to a lesser extent on p.
It is therefore essential to be able to very accurately measure the point—surface distance, and do so independently of the signal generated by the probe, in order to be able to separate the information relating to the position of the probe from the information relating to the local interaction with the surface. Preferably, this measurement must also be rapid, especially if it has to allow for a servocontrol in order to perform a scan at constant distance.
As described above, it is also necessary to accurately determine the relative orientation of the end of the probe relative to its directions of displacement: the geometrical faults of the probes mean that there is always a difference of orientation between the axes of the displacement and the geometrical axes of the probe. In the particular case of SECM, where the probe usually has a flattened point, in the form of a disk, with a conductive region at its center, the inclination of the probe determines the minimum distance that can be reached between this conductive region and the surface, which in turn limits the quality of the signal acquired.
Thus, an effective implementation of different scanning probe microscopy techniques requires the accurate and rapid positioning of a nano-object (the point of the probe) relative to a planar surface. Currently, that is obtained by methods specific to each technique. In the particular case of SECM, these techniques are particularly complex to implement and do not give full satisfaction; on this, see the article by M. A. O'Connell and A. J. Wain “Combined electrochemical-topographical imaging: a critical review” Anal. Methods 2015, 7(17), 6983-6999.
There is therefore a need for a technique for positioning an SPM probe, and more particularly an SECM probe, which is simpler and more effective than the methods of the prior art, particularly with a view to servocontrol allowing a scan at constant height.
Quite similar problems arise in the implementation of other micro- or nano-analysis techniques (using, for example, nano-pipettes) and/or micro- or nano-fabrication techniques (using, for example, SECM probes to induce localized electrochemical reactions).
The problem of accurate positioning of a micro- or nano-object also arises in the field of the fabrication of vertical heterostructures of two-dimensional materials (2D), or “Van der Waals nanostructures”, which are three-dimensional nanostructures obtained by stacking two-dimensional nano-objects, such as leaves of graphene or of similar materials, on a solid surface. Recent works have shown that such stackings can make it possible to obtain electronic or optoelectronic devices such as tunnel-effect diodes, vertical transistors, light emitters or detectors; see for example the article by L. Britnell et al. “Strong Light-Matter Interactions in Heterostructures of Atomically Thin Films”, Science 2013, 340, 1311 and that of T. Georgiou et al. “Vertical Field Effect Transistor based on Graphene-WS2 Heterostructures for flexible and transparent electronics”, Nature nanotechnology 2013, 8(2), 100-103. The stacking can be obtained by successive transfers: the leaves are first of all fixed to macroscopic pads which are positioned using micromanipulators, then released by chemically modifying or by dissolving the pads. The positioning is a critical operation, because the two-dimensional leaves are practically invisible which makes any visual control difficult. It is therefore necessary to proceed through trials, which is very ineffective and greatly limits the number of leaves that can be superposed. Contrary to the case of the scanning microscopy probes, where the most critical parameter is the probe—surface distance, in the case of the assembly of vertical heterostructures of 2D materials it is above all the position and the relative orientation (azimuth) of the nano-objects in the plane of the surface which has to be determined accurately. It is also very important to ensure the parallel alignment between the pads and the deposition surface, because even a small inclination can prevent contact between the leaves to be assembled.
The invention aims to remedy these drawbacks of the prior art, and more specifically to allow the positioning of a micro- or nano-object above a planar support by displacement performed under visual control.
According to the invention, this aim is achieved by virtue of the use of an optical technology known by the acronym BALM (Backside Absorbing Layer Microscopy), described in the following documents:
Ausserré, D., Amra, C., Zerrad, M., & Khachfe, R. A. (2014) “Absorbing Backside Anti-reflecting Layers for high contrast imaging in fluid cells” (No. arXiv: 1405.7672);
WO 2015/055809;
WO 2015/121462.
This technique consists in using a thin absorbent layer (often metallic), deposited on the surface of a transparent substrate, as contrast-amplifying antireflection layer. A sample deposited on the absorbent antireflection layer, even as thin as an atomic monolayer, appears with a high contrast when it is lit and observed through the substrate.
The article by D. Ausserré et al. “Absorbing Backside Anti-reflecting Layers for high contrast imaging in fluid cells”, Journal of Nanomedicine and Nanotechnology, Vol. 05, No. 04, Jul. 21, 2014 also describes absorbent antireflection layers and their application to the amplification of contrast in fluid cells.
The present inventors have realized that the interferential phenomena causing the antireflection behavior of the absorbent layer are also affected by the presence of a micro- or nano-object situated in immediate proximity to said layer, but not necessarily in direct contact therewith, which allows for a visual control of the positioning of the micro- or nano-object above the layer. In fact:
The contrast-amplifying effect of the absorbent antireflection layer makes the object easily visible through the substrate, without the observation being disturbed by the presence of micromanipulators; that allows for its accurate positioning in the plane of the layer.
The image of the object observed through the substrate is very sensitive to the distance relative to the absorbent antireflection layer; that makes it possible to measure this distance, and therefore to accurately position the object in a direction at right angles to the layer.
In the case of a micro- or nano-object having a locally planar surface (nanodisk at the end of an SECM probe, leaf of two-dimensional material, etc.), the image observed through the substrate is also very sensitive to the orientation of this surface relative to the absorbent antireflection layer; that makes it possible to ensure the parallel alignment (or obtain a desired inclination) between the object and the layer.
The abovementioned document WO 2015/121462 discloses the combined use of an absorbent antireflection layer, of an optical microscope lighting and observing this layer through the transparent substrate and a scanning probe microscope arranged on the side of the layer opposite the substrate. This document concerns only the study of a chemical reaction both by optical means and by a localized electrochemical measurement; it does not concern using the microscope to facilitate the positioning of the local probe.